Thunderstorm measurement

1. MAUSAM, 63, 2 (April 2012), 299-318
551.509.316 : 551.515.4 : 551.508.855
(299)
Development of nowcasting technique and evaluation of convective indices
for thunderstorm prediction in Gangetic West Bengal (India)
using Doppler Weather Radar and upper air data
DEVENDRA PRADHAN, U. K. DE*, U. V. SINGH**
India Meteorological Department, Kolkatta, India
*School of Environmental Studies, Jadavpur University, Kolkata, India
**Storm Detection Radar, India Meteorological Department, Sriganganagar, India
(Received 8 September 2010, Modified 2 November2011)
e mail : pradhandev1960@gmail.com
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ABSTRACT . Thunderstorm and hailstorm are well known short term severe weather phenomena which
sometimes turn in to natural hazard especially in Gangetic West Bengal region of India. Large vertical extent of the
cumulonimbus cloud, very high reflectivity, squally wind speed sometimes exceeding 100 km/h and heavy rainfall are the
main features of these thunderstorms during pre-monsoon period in this region. A study of 70 thunderstorms has been
carried out during the pre-monsoon season (March-May) of the year 2005 around Kolkata (22.5° N, 88.5° E) using
Doppler Weather Radar and Upper air data. Standard convective indices like CAPE, CINE, LI, BRN and VGP have been
evaluated and analyzed statistically. As no definite thresholds of the convective indices are available for thunderstorm
prediction in this region, an attempt has been made to find threshold of these indices for possible occurrences of
thunderstorms in Gangetic West Bengal region after the analysis of the thunderstorms during year 2005. The validity of
these convective indices has been checked with 34 occurrences of thunderstorms during 2006-2007 recorded by Doppler
Weather Radar Kolkata. The study reveals that nowcasting of thunderstorms may be done at least 2-3 hrs in advance with
a fair degree of accuracy using Doppler radar products only. However, the lead time of nowcasting may be further
improved if the convective indices are also analyzed and used in addition to the DWR data. A simple technique has been
suggested by the authors for better prediction of thunderstorms at least three to four hours in advance.
Key words - Doppler Weather radar (DWR), Gangetic West Bengal (GWB), Convective Available potential
Energy (CAPE), Convective Inhibition Energy (CINE), Lifted Index (LI), Bulk Richardson Number
(BRN), Vorticity Generating Parameter (VGP).
1. Introduction
Thunderstorm is a meso-scale system of space scale
ranging from a few kilometers to a few hundreds of
kilometers and time scale of less than an hour to several
hours. A severe thunderstorm includes some of the
attributes among heavy rain shower, lightning, thunder,
hail storm, dust-storm, surface wind squall, down-burst
and tornado. During the pre-monsoon season of March,
April and May, Gangetic West Bengal (GWB) and

2. 300 MAUSAM, 63, 2 (April 2012)
surrounding areas get severe thunderstorms called
“Norwester” also termed locally as “Kalbaisakhi”. Typical
features of these Norwesters are squally wind with speed
exceeding 100 km/h, heavy rainfall at the rate more than
100 mm/h and sometimes accompanied with hailstorm of
hail size more than 30 mm in diameter. It is well
documented that many people die every year due to
lightning strike, damaging winds flash flood arising due to
intense rainfall associated to the norwesters. De and Sen
(1961), De (1963), Mukherjee et al., (1972) and
Narayanan and Krishnamurthy (1966) and many other
authors have reported the results of their studies of these
thunderstorms, particularly during the pre-monsoon
season. These studies indicated that a large no of severe
thunderstorms move towards Calcutta (now renamed as
Kolkata) from NW direction during pre-monsoon season
every year. The details of the damage caused by the
thunderstorms have also been recorded in these studies.
Alvi et al., (1966), Raman and Raghvan (1961), Rao et
al., (1971), Koteshwaram and Srinivasan (1958) and
Krishna Rao (1966) have studied the role of conditional
Instability (CAPE) in the outbreak of severe
thunderstorms. These studies showed that the convective
indices play a key role in the prediction of instability and
genesis of thunderstorms. Earlier radar observations had
played a vital role in understanding of severe storms like
time taken to develop and dissipate and its propagation,
particularly about Kalbaisakhi qualitatively. As these
radars were not digitized, quantitative information on
rainfall associated with thunderstorm was not available.
1.1. Role of Doppler weather radar in thunderstorm
studies
The development of Doppler Weather Radar (DWR)
has provided a very good quantitative analysis of severe
weather phenomena along with several derived products
useful for research purpose. The research field programs
(Staff, 1979, Doviak et al., 1979, Wilson et al., 1980)
showed that significant nowcasting information can be
obtained from Single Doppler radar. This has been
accepted worldwide by the meteorological community
that DWR is an indispensable tool for the study of severe
weather phenomena like thunderstorm, hailstorm, tornado,
cyclone (hurricane) etc. Presently the identification of
Tornadoes, gust fronts, downbursts and fronts can be
easily accomplished by the forecasters monitoring and
interpreting the DWR products. It is one of the most
useful observational tools for the detection of
thunderstorm cell, to monitor their development and to
predict their movement. Three base products available
from DWR are :
(i) Radar Reflectivity (Z) : The amplitude of the radar
returned echoes is directly proportional to the sixth power
of the rain drop diameter and therefore this parameter is
mainly used for the quantitative estimation of precipitation
contents, liquid water content, rainfall rate associated with
a thunderstorm, vertical and horizontal developments of
the cloud and to study the three dimensional structure of a
thunderstorm cell.
(ii) Radial Velocity (V) : Using Doppler principle for
electromagnetic waves, the radial component of the
velocity of winds may be estimated by this parameter.
Derived products from radial velocity data provide
information for the direction of movement of the system,
horizontal components of the wind velocity and wind
shear. It has been well documented by Doviak et al.,
(1979) and Wilson and Wilk (1981) that the radial wind
information obtained from a single DWR can be used to
derive and estimate the horizontal wind, wind shear and
other products useful for the study of severe weather
phenomena.
(iii) Spectrum width (W) : The spectrum of variability of
radial velocity of the individual drops with respect to the
mean radial velocity of the system is called spectrum
width. It is an indicator of turbulences associated with the
winds and thus turbulent environment.
Many products have been derived from these basic
data using standard algorithms approved by the
meteorological scientists. In this paper, capabilities of
DWR have been analyzed to develop a nowcasting
technique for the occurrence and movement of
thunderstorms in the Gangetic West Bengal region. A
statistical study of 70 thunderstorms which occurred
during pre-monsoon period of the year 2005 has been
performed using reflectivity and velocity data generated
by DWR Kolkata along with upper air data from the
meteorological office Kolkata to find a range of
convective indices for prediction of development of
thunderstorms. Thunderstorms during 2006 and 2007 have
also been recorded and analyzed for validation of
nowcasting technique and proposed convective indices.
1.2. Typical features of norwesters of north east
India
The months of April and May account for most of
the thunderstorm development. A majority of these
thunderstorms are accompanied by squalls and the
frequency of squalls increases from northwest to
southeast. The squalls are mostly from northwesterly
direction. The highest wind speed recorded in these
squalls reached up to 140-150 km/hr. A definite time
sequence had been observed for occurrence and
movement of these Norwesters by various authors in
India. They generally develop over Bihar plateau,

3. PRADHAN et al. : THUNDERSTORM PREDICTION USING DWR 301
southeast Madhya Pradesh or West Orissa and travel east
or southeastwards towards Gangetic West Bengal or head
Bay of Bengal. They do not advance more than 130 to 160
km into the sea. Desai (1950) mentioned that
thunderstorms approach most frequently from west to
northwest and their occurrence takes place during 1200
UTC and 1500 UTC with a movement speed of 50 to 60
km/hr. The intensity of echoes received by analog radars
were quite strong and showed qualitatively the potential of
heavy rainfall. The vertical extent of such thunderstorm
clouds exceeded 18 km in many cases but on few
occasions crossed 20 km.
1.3. Classification of storms
To discuss the various aspects of cloud cells
movement, authors have classified the storms in the
following categories :
1.3.1. Based on structure
(a) Single cell storm : A single cloud cell moving
independently without combining with any other cell has
been considered as a single cell storm in this study.
Usually the life cycle of such storms is between
30 minutes to 1 hour as studied by many authors in India,
however, through DWR observations it was found that
many single cell thunderstorms lived for more than 1
hour.
(b) Multiple cell storms : This storm system consists of
a series of evolving cells. Cells typically form on or near
the storm periphery at 10-15 minutes intervals and each
cell eventually becomes the dominant cell of the storm
complex, building to higher levels as it approaches and
finally merges with the main storm complex. Precipitation
forms in the new updraft and is held aloft temporarily
while the cell matures. Finally, precipitation unloads as
the cell matures resulting in a heavy gush of rain. The
cluster of cells formed by the combination of neighboring
cells and the whole system moves together is taken as a
multiple cell storm. The duration of such storms has been
found to be 3-6 hrs.
(c) Squall line : A chain of thunderstorms connected
together and moving like a single entity, usually
associated with a very high speed wind. As per the
literature available, the minimum width to length ratio for
a squall line should be 1:10.
1.3.2. Based on DWR estimated reflectivity and
vertical extent
(a) Severe thunderstorm : Thunderstorm in which the
height of 50 dBz reflectivity is more than 16 km has been
considered as severe thunderstorm. Usually multiple cell
thunderstorms generate severe thunderstorms, however,
the possibility of single cell thunderstorms to become
severe may not be ruled out. It has been observed from the
rainfall data that such thunderstorms provide heavy
rainfall at the rate of 30 mm/hr or more.
(b) Moderate thunderstorm : Thunderstorm in which the
height of 50 dBz reflectivity lies within 12 km and 16 km
has been considered as moderate thunderstorm. The study
of the rainfall data revealed that such thunderstorms
produce rainfall in the range of 5- 30 mm/hr.
(c) Weak thunderstorm : Thunderstorm in which the
maximum reflectivity lies below 40 dBz has been
considered as weak thunderstorm. The vertical extent of
weak thunderstorms may reach up to 10 km where the
reflectivity is not more than 20 dBz. Such thunderstorms
may be accompanied with thunder and lightning but
provide a rainfall in the range of 1- 5 mm/hr.
(d) Very weak thunderstorm : Thunderstorm in which
the maximum reflectivity lies below 30 dBz has been
considered as weak thunderstorm. The vertical extent of
such thunderstorms does not exceed 6 km and they
dissipate very quickly but produce thunder and lightening
for a short interval. A drizzle or very light rain has been
observed with such thunderstorms.
2. Data
India Meteorological Department (IMD) installed
two DWR's at Chennai & Kolkata during the year 2002
(August & October respectively) and two more radars in
the year 2005 at Machilipatnam and Visakhapatnam (June
& Sept). All these DWR stations are located on the East
Coast of India for a continuous surveillance of severe
weather system in Bay of Bengal. These are S-Band
radars (10 cm wavelength) operating at 2875 MHz,
transmitting a peak power of about 750 KW using
Klystron as amplifying device. The unambiguous range
for reflectivity measurement in surveillance mode is 400
km and maximum unambiguous radial wind estimates
(using velocity unfolding technique) within 250 km radius
is 64 m/s (230 km/hr). With the installation of these radars
along east coast of the Bay of Bengal, round the clock
weather observation and analysis capabilities of IMD have
been enhanced to a great extent. DWR generates
voluminous data related to observed weather phenomena
by performing a volume scan of duration 15-20 minutes.
The time for a volume scan may be reduced to 10 minutes
or even less as per requirement of the data.
A sequence of reflectivity and radial velocity data of
more than 100 thunderstorms has been collected at every

4. 302 MAUSAM, 63, 2 (April 2012)
20 minutes interval during pre-monsoon period (March-
May) of the years 2005-2007 and analyzed statistically.
Some of the standard convective indices for the
corresponding days are also evaluated for the purpose of
validation. Rainbow 3.4 software has been supplied by the
manufacturer for deriving the products from the base data.
A brief of two products Max_Z and VVP_2 of Rainbow
software used in this radar by M/S Gematronik, Germany
are given below :-
2.1. Max_Z (Maximum Reflectivity)
This is a derived product from volume reflectivity
data, which gives a three dimensional view of the cloud.
The algorithm calculates each vertical column of the cloud
and finds the maximum reflectivity in that column. The
reflectivity is presented in pseudo colours and scale is
shown to the right of the product. The overall three
dimensional cloud is presented in the form of a composite
of three partial images in to a single imagery. The partial
images are:-
(i) A top view as the highest measured reflectivity
values in Z - direction. This image shows the highest
reflectivity value for each vertical column seen from the
top of the cartesian volume.
(ii) A north-south view of the highest measured
reflectivity values in Y - direction. This image is appended
above the top view and shows the highest measured
reflectivity value for each horizontal line scan from north
to south. The height of the cloud can be observed with
respect to reflectivity from this image.
(iii) An east-west view of the highest measured values in
X - direction. This image is appended to the right of the
top view and shows the highest measured reflectivity
value for each horizontal line scan from east to west. This
image is also an indicator of height of the cloud and
avoids the overlapping of the clouds in N-S projection if
any.
2.2. VVP_2 (Volume Velocity processing)
Waldtenfel and Corbin (1979) proposed a technique
to derive horizontal winds from radial velocity observed
by single DWR over a limited area of about 50 km around
the radar. A vertical profile of horizontal winds up to 7.5
km may be derived at equal steps selectable by users. This
product displays the horizontal wind velocity and the wind
direction in a vertical column of the radar site. Authors
have compared the winds derived from Doppler Weather
Radar with the RS/RW data and found that there is a close
agreement in both. This means that for local observations
for winds over an area of 50 km this product (VVP_2)
may be taken as a standard product. A linear wind field
model is used to derive the additional information from
the measured radial velocity data. An algorithm for
evaluating the product from radial component of the
velocity is described below for better understanding. The
algorithm calculates velocity and wind direction for a set
of equi-distant layers.
2.3. Convective Indices
2.3.1. CAPE (Convective Available Potential
Energy)
It is the positive area on the sounding between the
level of free convection (LFC) and the equilibrium level
and expressed in J/kg. As per the studies performed by
Stensrud et al. (1997), following ranges have been
proposed as an indicator of instability criterion :
0 to 999 - Marginal
Instability
: 1,000 to 2,500-
Moderate Instability
2,500 to 4,000 Strong
Instability
: > 4,000 Extreme
Instability
Stensrud et al., (1997) also stated that
any thunderstorm that develops in an environment
> 4,000 J/kg should be considered extremely hazardous.
However, it is just the available energy for a parcel to rise
above due to convection and cannot be taken as a true
measure of instability. It can vary drastically on the
mesoscale depending on surface heating, moisture, and
thermal advection.
2.3.2. CINE (Convective Inhibition Energy)
Convective inhibition is the negative area on a
sounding found between the lowest condensation Level
(LCL) and LFC and is expressed in J/kg. Rassmussen and
Blanchard (1998) suggested some threshold values from
the studies performed over several hundreds of
thunderstorms in US.
< 25 J/kg associated with significant tornadoes.
~ 50 J/kg associated with moderate echoes.
> 100 J/kg precludes thunderstorm development
without significant forcing
2.3.3. BRN (Bulk Richardson Number)
Through this index an attempt is made to predict the
storm type by balancing instability and shear (Strensrud

5. PRADHAN et al. : THUNDERSTORM PREDICTION USING DWR 303
et al., 1997, Davies 1998). The various limits available in
the literature are as follows :
40 to 140 m2
/s2
: potential for significant
supercells.
35 to 40 m2
/s2
: potential for marginal
supercell events.
< 40 m2
/s2
: associated with outflow
dominated storms.
Weisman and Klemp (1984) have quoted this index
as good discriminator between storm types, but
Rasmussen and Blanchard (1998) refute this.
2.3.4. LI (Lifted Index)
It is defined as difference between a given saturated
parcel temperature and the 500 hPa environmental
temperature (0° C).
0 to -2° C
(Weak Instability)
: -3 to -5° C
(Moderate Instability).
-6 to -9° C
(Strong Instability)
: < -9° C
(Extreme Instability).
The more negative the number, the more unstable is
the environment.
2.3.5. VGP (Vorticity Generation Parameter)
This index is calculated by the formula (Rasmussen
and Blanchard, 1998)
VGP = [S (CAPE) 1/2
] where S is the hodograph
length/depth.
When the value of VGP exceeds 0.2, it indicates
increasing possibility of tornadic supercell. This index is
associated with physical concept of tilting of the vorticity
axis.
Values of the convective indices as suggested by
various authors vary from place to place and season to
season and therefore, cannot be taken as standard for the
area under study. Ludlam (1980) showed that when
vertical wind shear (VWS) is very high, the updrafts and
downdrafts of the thunderstorm become so organized that
they do not interfere with each other, with the result that a
severe thunderstorm is formed realizing the maximum
potential of Convective Available Potential Energy
(CAPE). Chaudhary (2006) conducted a study using
technique called “Ampliative reasoning”, to see if
minimization of CINE or maximization of CAPE is
pertinent for the genesis of severe thunderstorms of pre-
monsoon season over North East part of India. The study
revealed that for the genesis of severe thunderstorms in
Gangetic West Bengal region, minimization of CINE is
fundamental, whereas neither very high nor very low
value of CAPE is of any significance. As suggested by
Chaudhary (2006), CAPE value of 7000 J/kg and CINE
value 100 J/kg will support the occurrence of severe
thunderstorms in Gangetic West Bengal region.
3. Methodology
3.1. DWR Data
Thunderstorms in Gangetic West Bengal region
during pre-monsoon period usually follow a fixed pattern
based on the thermodynamic instability and the wind
direction. Since speed and direction of wind around
Kolkata are always available from the DWR derived
products like Plan Position Indicator (PPI_V) and VVP_2,
forecasters can easily issue the nowcasting by observing
the initiation and development of the system and its
direction of movement. An analysis of DWR recorded
observations of about 250 days during 01 March to 31
May, 2005, 2006 and 2007 has been performed in the
present work. More than 100 thunderstorms of various
types have been analyzed using DWR data for intensive
study. Convective indices of the same period have also
been evaluated and analyzed. Based on the analysis of the
year 2005 data, a range of convective indices has been
proposed for the prediction of severe and moderate
thunderstorms in GWB region. The upper air and DWR
images during the years 2006 and 2007 have been
analyzed for the validation of the results obtained from the
analysis of year 2005 and to validate the proposed range
of convective indices.
3.2. Upper air data and convective indices
Upper air data of 0000 UTC and 1200 UTC for all
thunderstorm days has been collected from
Meteorological Office Kolkata. Using RAOB software
(The RAwinsonde Observation Program from
Environmental Research Services, Shreeji and Company
for automated drawing of T-Phi gram and derivation of
convective indices), some of the standard convective
parameters CAPE, CINE, LI, VGP and BRN have been
derived. These parameters are studied in conjunction with
actual occurrences of thunderstorms recorded by Doppler
Weather Radar, Kolkata. Based on the analysis of
convective indices observed on a thunderstorm day and
the actual occurrences of thunderstorm, threshold values
for severe and moderate type of thunderstorms have been

6. 304 MAUSAM, 63, 2 (April 2012)
TABLE 1
(Severe thunderstorms during 2005 and convective indices)
Date CAPE CINE LI VGP BRN
Time & sector of
initiation (UTC)
Time of
Dissipation (UTC)
Duration/ Direction
of movement
23 Mar 2005 4257 0 -6.4 0.674 356 0504/ NW 1104 6 hrs/SE
10 Apr 2005 4618 0 -6.1 0.599 242 0832/ NW 1232 4 hrs/ SE
19 Apr 2005 3989 -5 -5.8 0.856 286 0727/NW 1147 4 hrs 20 mts/ SE
05 May 2005 4730 (12Z) -10 -7.2 0.665 195 0748/NE 1328 5 hrs 40 mts/ W
17 May 2005 5290 0 -6.9 0.754 214 0418/NNW 1201 7 hrs 43 min/ SSE
19 May 2005 6317 -8 -13.1 0.329 553 0821/NW 1221 4 hrs/NW
22 May 2005 5604 (12Z) 0 -10.3 0.770 272 0921/NW 1341 4 hrs 20 mts/ SE
TABLE 2
( Moderate thunderstorms during 2005 and convective indices)
Date CAPE CINE LI VGP BRN
Time & sector of
initiation (UTC)
Time of
Dissipation (UTC)
Duration/ Direction of
movement
11 Mar 2005 2889 -25 -8.3 0.419 180 0827/SW 1207 3 hrs 30 min/E
18 Mar 2005 1955 -51 -5.9 0.240 87 0934/NW 1114 1 hr 40 min/SE
19 Mar 2005 (12Z) 2476 -31 - 0.421 120 0914/NW 1334 4 hrs 20 min/SE
24 Mar 2005 3567 -58 -5.8 0.567 187 0424/SW 0924 5 hrs/SE
30 Mar 2005 (12Z) 2897 -89 -6.9 0.331 56 0922/NW 1442 5 hrs 20 min/E
29 Apr 2005 4570 -47 -7.0 0.578 71 0814/SW 1334 5 hrs 20 min /E
03 May 2005 (12Z) 3526 0 -5.6 0.392 57 1228/NW 1848 6 hrs 20 min/SE
04 May 2005 602 -101 -3.1 0.181 13 0828/NW 1328 5 hrs /SE
06 May 2005 3996 -103 -3.9 0.23 33 0808/NW 1348 5 hrs 40 min/SE
TABLE 3
(Local Thunderstorms during 2005 and convective indices)
Date CAPE CINE LI VGP BRN
Time & sector of
initiation (UTC)
Time of dissipation
(UTC)
Duration/ Direction of
movement
16 Mar 2005 3135 -3 -6.5 0.395 209 0744/E 0904 01 hr 20 min/W
02 May 2005 556 -154 0.2 0.133 208 1022/NE 1208 01hr 46 min/E
proposed for the prediction in GWB region (Tables 6 &
7). These indices have not been proposed so far in this
region for the nowcasting of deadly thunderstorms
(Norwesters) and these were very much needed for
forecasters to issue timely prediction of heavy rainfall and
squally winds at least 2-3 hrs in advance.
4. Analysis
4.1. Analysis of thunderstorms during year 2005
DWR Kolkata has recorded 70 thunderstorms during
01 March to 31 May 2005 around Kolkata within a radius

7. PRADHAN et al. : THUNDERSTORM PREDICTION USING DWR 305
of 300 km. These thunderstorms have been categorized as
severe, moderate, weak and very weak thunderstorms
under the definitions given above. Another classification
has also been done on the basis of multiple and single cell
thunderstorms.
In the present study, only severe and moderate
thunderstorms have been analyzed as their damaging
potential is very severe. The data collected during year
2005 depicts that during May 16 days, April 11 days and
March 12 days of thunderstorm activities took place. Data
also shows that during the months of March and May, on
an average two thunderstorms were developed on the
thunderstorm day. The analysis shows that out of 70
thunderstorms, 55 were multiple cell type and 15 were
single cell type. Multicellular storms were of moderate
and severe categories whereas single cell thunderstorms
have been found to be all types including weak and very
weak thunderstorms. The height and reflectivity analysis
showed that vertical extent of some of these
thunderstorms exceeded more than 20 km and reflectivity
more than 60 dBz. The great vertical extent confirms that
the updrafts associated with severe thunderstorms in this
region are very strong.
Table 1 shows the analysis of severe thunderstorms,
indicate that duration of severe thunderstorms varies from
2 hrs to 6 hrs. Thunderstorm on March 23, which arrived
from NW direction, traveled a distance of more than 300
km and lasted for 6 hrs. This thunderstorm combined with
another multiple cell thunderstorm, coming from SW
direction developed slightly later and produced a squall
line of length exceeding 400 km (Fig. 1). On several other
occasions the multiple cell thunderstorms generated
“Squall lines” associated with strong gusty winds of the
order of 100 km/hr or more. On 10 April and 05 May,
hailstorms occurred along with severe thunder, lightning
and heavy rainfall. Many other thunderstorms may also be
associated with hailstorms but due to lack of awareness in
the general public and non-reporting by media, they
remained unnoticed. The analysis also shows that all
severe thunderstorms in the year 2005 arrived from NW
direction except one on 05 May 2005 which came from
NE direction towards Kolkata. Some of the moderate
category thunderstorms which were generated in the SW
and traveled towards Kolkata are presented in Table 2. As
stated by many scientists in their studies that energy
redistribution takes place between the adjacent cells
during their movement, the same has been confirmed by
consecutive DWR observations as degeneration and re-
generation of cells could be seen very clearly during their
propagation.
4.1.1. Analysis of multiple cell thunderstorms
Out of 55 multiple cell thunderstorms observed by
DWR, 29 thunderstorms moved towards Kolkata and
arrived within 50 km range before dissipation (52.7%).
The sector wise statistics of thunderstorm cells developed
and hit Kolkata is given below :
(i) NW direction, 18 hit out of 24 developed (75%).
(ii) SW direction, 07 hit out of 20 developed (35%).
(iii) SE direction, 03 hit out of 05 developed (60%).
(iv) NE direction, 01 hit out of 06 developed (16.6%).
4.1.2. Analysis of single cell thunderstorms
Out of 15 single cell thunderstorms, 07
thunderstorms arrived Kolkata (48%). Sector wise
statistics is given below:-
(i) NW direction, 03 hit out of 07 developed (42.8 %).
(ii) SW direction, none hit out of 03 developed (00 %).
(iii) SE direction, 01 hit out of 01 developed (100 %).
(iv) NE direction, 03 hit out of 04 developed (75 %).
4.1.3. Analysis of locally generated thunderstorms
As Kolkata is about 100 km away from the coast,
some thunderstorms were locally generated in the vicinity
of Kolkata. Table 3 shows detailed analysis of locally
generated thunderstorms. Most of these storms were
single cell moderate thunderstorms which moved around
the place of origin and did not move far away. Their life
span was not more than 2 hrs but provided heavy rainfall
over a small area creating heavy damage to crops as
reported by media.
4.1.4. Analysis of squall lines
Table 4 shows the analysis of squall lines formed
during the period of analysis. Out of 8 squall lines
generated in the region, 5 were formed (62.5%) due to
combination of multiple cell thunderstorms developed in
both NW and SW sectors. The time of formation was
different in these sectors but they moved almost together
with the same speed and generated squall line
configuration. 3 Squall lines were formed (37.5%) due to
multiple cell thunderstorms in NW sector only. No squall
line was formed when thunderstorms were developed in
other sectors. Max_Z pictures of squall lines analyzed in

8. 306 MAUSAM, 63, 2 (April 2012)
TABLE 4
(Squall lines during 2005)
Date Time of formation Length of squall line Speed of movement Sector of formation Direction of movement
23 Mar 2005 1124 UTC 400 km 60 km/h SW+NW SE
24 Mar 2005 0724 UTC 250 km 55 km/h SW+NW E
30 Mar 2005 1202 UTC 200 km 50 km/h NW SE
29 Apr 2005 1234 UTC 175 km 50 km/h SW+ NW SE
05 May 2005 1328 UTC 200 km 50 km/h NE W
06 May 2005 1308 UTC 175 km 50 km/h NW SE
17 May 2005 0921UTC 200 km 50 km/h SW+NW E
19 May 2005 1101 UTC 200 km 50 km/h SW+NW SE
TABLE 5
Average wind pattern estimation during 2005
Height March April May
0.3 km S/SW S/SW S/SW
1.5 km W/SW WNW W/SW
2.1 km SW W NW
3.1 km SW W NW
TABLE 6
Proposed Convective indices for severe thunderstorms
CAPE CINE LI VGP BRN
4000-6000 -30 to 0 -6.1 to -10.3 > 0.5 200-400
TABLE 7
(Proposed Convective indices for Moderate Thunderstorms)
CAPE CIN LI VGP BRN
2000-4000 -100 to -30 -2.0 to -7.0 0.2 to 0.5 80-200
Table 4 are shown in Figs. 1 to 8. This is observed
that most of the squall lines were formed around 1200
UTC (23, 30 March, 29 April, 17, 19 May). The case of
05 May, was an exception when the thunderstorm arrived
from NE direction and lasted for 5 hrs 40 min.
4.1.5. Analysis of convective indices
The convective indices for all the category of
thunderstorms have been collected, tabulated and
analyzed. It is found that a fixed pattern and trend is
shown by these indices for moderate and severe
thunderstorms but no such values could be obtained for
locally generated thunderstorms and therefore, such
locally generated thunderstorms may not be predicted
accurately by these indices.
4.1.6. Analysis of VVP_2
The vertical profile of the wind (VVP_2) over
Kolkata provides an idea of the prevailing winds in the
region at various levels up to 7.5 km with a step of 0.3
km. The movement of the clouds may be estimated up to a
very good accuracy by this product. As evident from
Fig. 9, a strong S/SW wind at the lower levels around
Kolkata feeds the moisture from Bay of Bengal to the
environment which helps thunderstorm to develop.
Therefore, the probability of the development of a
thunderstorm over Kolkata may be high if the direction of
the winds at 0.3 km and 0.6 km is either Southerly or
south easterly with speed between 15-20 knots. This has
also been analysed that the setting of S/SE winds during
0600-0700 UTC period will have more probability of the
system formation in the North West sector and their
propagation towards Kolkata. Table 5 shows the variation
of the wind direction from March-May during the year
2005 as observed by VVP_2 pictures. The low level wind
at 0.3 km is mainly southerly or southwesterly during all
the months indicates the availability of moisture from the
Bay of Bengal because of the nature of the topography of
the region. During March and May the winds at 1.5 km
are Southerly or South-westerly whereas during April the
direction of wind was found to be North-NorthWesterly.
The genesis of thunderstorm indicates that during the
developmental phase of the cell, when the updraft is
predominant, the transfer of horizontal momentum to
upper layer may cause the movement to be influenced by

9. PRADHAN et al. : THUNDERSTORM PREDICTION USING DWR 307
Fig 1. Squall line on 23 Mar 2005 (1124 UTC) Fig 2. Squall line on 24 Mar 2005 (0724 UTC)
Fig 3. Squall line on 30 Mar 2005 (1202 UTC) Fig 4. Squall line on 29 Apr 2005 (1234 UTC)
Fig 5. Squall line on 5 May 2005( 1328 UTC) Fig 6. Squall line on 06 May 2005 (1308 UTC)

10. 308 MAUSAM, 63, 2 (April 2012)
the lower level wind pattern; but once the cell attains
maturity the down draft transportation of momentum
ensures that the movement is governed by the winds at 3-4
km levels.
4.1.7 Wind squall and Hailstorm during year 2005
Table 8 shows the occurrences of Wind squall and
Hailstorm as recorded by the Meteorological observatories
during year 2005 in the neighborhood of Kolkata and
adjoining areas of Odisha(earlier named as Orissa). It is
observed that meteorological observatories recorded the
events (Wind squall/ hailstorm) for 13 days only. This is
possible that the wind squall might have occurred at some
different places outside the observatory area and therefore
could not be recorded. Table 8 depicts that on 23 March
2005, Wind squall at speed 49 km/hr from NW direction
was recorded by Alipore at 1205 UTC. Fig. 1 is the DWR
recorded image of Squall line at 1124 UTC arriving from
NW direction, when it was about 40 km away from
Alipore (Kolkata). The linear stretch of the line was 400
km and was traveling at the speed of about 55 km. It
means that during next 40 minutes (1205 UTC) when the
distance of squall line was approximately 5-10 km,
squally winds from NW direction were recorded by
Alipore Met. Office. It is well known fact that the wind
squall is originated from a Cb cloud ahead of the
thunderstorm over that region, the same has been
confirmed by DWR image and the observation by Alipore
observatory.
This may be seen from the Tables 1, 2 & 8 that wind
squall was reported by the different stations on those days
when there was a severe or moderate type of thunderstorm
was formed. Hailstorms were also reported by the
observatories and listed in Table 8.
4.2. Analysis of thunderstorms during 2006
During 26 April, 2006 to 15 May, 2006, DWR
Kolkata was unserviceable hence no observation could be
available during that period. The study of 14 moderate and
severe thunderstorms during rest of the period has been
performed along with convective indices of corresponding
days. It was observed that only two severe and one
moderate thunderstorm developed during March 2006.
Thunderstorms were associated with the hailstorms on 10
March, 08, 10 & 13 April whereas Squall lines were
formed on 22 April and 30 May. As proposed in Tables 9
and 10 for severe and moderate thunderstorms, it was
found that CAPE and CINE agree very closely for almost
all cases except on 08 April, 2006 when the CAPE and
CINE values were on the higher side and BRN was lower.
On an average the values of convective indices lie in the
range and substantiate to the proposed ranges of
convective indices (Tables 9 & 10). Comparison of severe
and moderate thunderstorms from Tables 9 and 10 showed
that duration of severe thunderstorm was much higher
than those of moderate types (except on 31 May, 2006).
The VVP_2 product (Fig. 11) of 13 April, 2006 shows a
Fig 7. Squall line on 17 May 2005 (0921 UTC) Fig 8. Squall line on 19.May 2005 (1101 UTC) Reflectivity Scale

11. PRADHAN et al. : THUNDERSTORM PREDICTION USING DWR 309
TABLE 8
Wind squall/hailstorm recorded by the Meteorological observatories during year 2005
Date Squall Time
(UTC)
Station Squall
Speed/direction
Duration of
squall
Time (UTC)/
Duration of hailstorm
Size of
hailstones
Station
reported
11 Mar 2005 0726 Malda 55 km/h (W) 2 min Nil Nil Nil
19 Mar 2005 Nil Nil Nil Nil 0900 /10 min 2 cm Bankura
23 Mar 2005 1205 Alipore 49km/h (NW) 3 min Nil Nil Nil
30 Mar 2005 1130 Alipore 64 km/h (NW) 1 min Nil Nil Nil
10 Apr 2005 1300
1635
2000
1315
Alipore
Alipore
Alipore
Dumdum
59 km/h (NW)
55 km/h (S)
59 km/h (SW)
50 km/h (NW)
2 min
2 min
1 min
2 min
Nil Nil Nil
19 Apr 2005 1410 Bhubaneshwar 58 km/h (NNE) 2 min Nil Nil Nil
29 Apr 2005 Nil Nil Nil Nil 1647/4 min 4.4 cm Malda
02 May 2005 1127 Dumdum 90 km/h (S) 3 min Nil Nil Nil
03 May 2005 0910 Bhubneshwar 78 km/h (SSW) 2 min Nil Nil Nil
04 May 2005 1300
2350
Alipore
Alipore
76 km/h (NW)
76 km/h (NW)
1 min
1 min
1248 2 cm Alipore
17 May 2005 0820 Malda 55 km/h (N) 2 min Nil Nil Nil
18 May 2005 2000 Malda 56 km/h (NW) 3 min Nil Nil Nil
19 May 2005 Nil Nil Nil Nil 10 min 2 cm Sriniketan
TABLE 9
Severe thunderstorms during 2006 and convective indices
Date CAPE CINE LI VGP BRN
Time & sector of
initiation (UTC)
Time of
dissipation (UTC)
Duration/direction
of movement
15 Mar 2006 4896 0 -7.45 0.765 312 0912/NW 1142 2 hrs 30 min /SE
04 Apr 2006 4335 -23.5 -6.62 0 .564 236 0938/NW 1238 3 hrs/SE
08 Apr 2006 4434 0 -7.6 0.684 234 0908/NW 1238 3 hrs 30 min SE
10 Apr 2006 4235 -11 -6.8 0.646 288 0738/ NW 1308 5 hrs 30 min /SE
13 Apr 2006 4384 0 -9.0 0.503 446 0812/ NW 1322 5 hrs 10 min /E
24 Apr 2006 3598 0 -9.8 0.453 289 0827/ NW 1327 5 hrs SE
30 May 2006 3990 -5 -7.5 0.440 176 0934/ W 1440 5 hrs 06 min E
similar wind pattern as on 29 April, 2005 (Fig. 10) with
low level southerly winds and upper level northwesterly
winds and severe thunderstorms occurred both the days.
Fig. 12 shows that VVP_2 pattern on 24 April, 2006 was
somewhat different from other days when lower level
winds had the magnitude of 5 knots and upper level winds
were also not very strong, a severe multiple cell
thunderstorm system occurred that day which survived for
about 5 hours (Table 9). The system traveled more than
200 km before dissipation. Fig. 13 shows two strong
clusters of convective clouds having vertical development
more than 18 km penetrating tropopause. Fig 14 also
shows two severe thunderstorms in the vicinity. The cell
at 100 km distance NW from DWR produced a hailstorm

12. 310 MAUSAM, 63, 2 (April 2012)
TABLE 10
Moderate thunderstorms during 2006 and convective indices
Date CAPE CINE LI VGP BRN Time & sector of
initiation (UTC)
Time of dissipation
(UTC)
Duration/Direction
of movement
04 Mar 2006 3120 -56 -5.62 0.435 152 0924/ NW 1224 3 hrs /E
10 Mar 2006 3570 -88 -7.32 0.343 228 2119/ SW 2249 1 hr 30 min /SE
08 Apr 2006 (12Z) 4751 -112 -7.2 0.746 50 1438/ NW 1638 2 hrs/SSE
19 Apr 2006 2014 0 -5.9 0.249 28 0810/SW 1210 2 hrs/SE
22 Apr 2006 2585 -30 -8.2 0.492 245 0910/NW 1410 5 hrs/E
29 May 2006 1586 -5 -3.7 0.26 541 1221/NW 1414 1 hr 53 min /SE
31 May 2006 3666 -41 -9.5 0.564 102 1103/ NW 1534 4 hrs 31 min/ SE
TABLE 11
Wind squall/hailstorm recorded by the Meteorological observatories during year 2006
Date Squall
Time (UTC)
Station Squall
Speed/Direction
Duration
of Squall
Time(UTC)/Duration
of hailstorm
Size of
hailstones
Station reported
31 Mar 2006 Nil Nil Nil Nil 1120/15 min 1 cm Dumdum
04 Apr 2006 Nil Nil Nil Nil 1100 /5 min 1 cm Balasore
08 Apr 2006 1130 Bankura 56 km/h (NW) 2 min Nil Nil Nil
10 Apr 2006 1430 Malda 55 km/h (SW) 3 min 1132/3 min 2 cm Sriniketan
13 Apr 2006 1244
1220
Alipore
Dumdum
72 km/h
65 km/h
2 min
1 min
0125/20 min 1 cm Keonjhar
22 Apr 2006 1300
1258
Alipore
Dumdum
70 km/h (NW)
48 km/h (NW)
2 min
1 min
1230/5 min 0.7 cm Dumdum
24 Apr 2006 1032
1308
1255
Bankura
Alipore
Dumdum
56 km/h (N)
109 km/h (NW)
76 km/h (NW)
2 min
1 min
2 min
Nil Nil Nil
TABLE 12
Severe thunderstorms during 2007 and convective indices
Date CAPE CINE LI VGP BRN Time & sector of
initiation (UTC)
Time of dissipation
(UTC)
Duration/ Direction of
movement
22 Mar 2007 3564 -10 -7.3 0.165 269 1216/ NW 1717 5 hrs /
09 Apr 2007 2809 -19 -11.2 0.545 98 1444/ NW
1704/ NW
1904
2324
4 hrs 20 min /SE
6 hrs 20 min/E
12 Apr 2007 2318 -71 -9.1 0.375 54 0644/ NE
1000 /SW
1318
1338
6 hrs 34 min/W
3 hrs 38 min/NE
(Cells merged)
13 Apr 2007 4346 -14 -8.6 0.224 324 0806/ SW 1206 4 hrs /SE
26 Apr 2007 2924 -16 -10.8 0.43 100 0826/ SW 1226 4 hrs /E
18 May 2007 4653 0 -8.6 0.233 265 0928/ NW 1228 3 hrs /SE

13. PRADHAN et al. : THUNDERSTORM PREDICTION USING DWR 311
around 1140 UTC as reported by newspapers.
Fig. 15 is the Max_Z picture of the thunderstorm on 13
April, 2006 at 1122 UTC which also resembles with that
of 29 April, 2005 in terms of severity (Fig. 4). Values
of convective indices were also in agreement on these
days.
4.2.1. Wind squall and hailstorm during year 2006
Tables 9 & 10 depict the occurrence of severe and
moderate thunderstorms within 300 km radius from
Kolkata during the year 2006 and Table 11 shows the
Fig .9. VVP_2 picture of 23 Mar 2005 Fig 10. VVP_2 picture of 29 Apr 2005
Fig. 11. VVP_2 picture of 13 Apr 2006 Fig. 12. VVP_2 picture of 24 Apr 2006

14. 312 MAUSAM, 63, 2 (April 2012)
occurrence of wind squalls and hailstorms reported by the
meteorological observatories.
(a) On 04th
April, a severe thunderstorm arrived from
NW and travelled through SW sector of Kolkata and
lasted for 3 hrs. A hailstorm was reported by Balasore
(200 km SW from Kolkata) at 1100 UTC with 1 cm
hailstones and lasted for 5 minutes.
(b) On 08th
April a severe thunderstorm lasted for 3 hrs
30 min arrived from NW direction during 0908-1238
UTC. Meteorological Office at Bankura (located at 150
km NW from Kolkata) reported wind squall of the
magnitude 56 km/hr at 1130 UTC.
(c) On 10th
April a severe thunderstorm travelled from
NW and lasted for 5 hrs 30 min during 0738-1308 UTC.
Fig. 13. Thunderstorm on 08 Apr 2006 (1208 UTC)
Fig. 14. Thunderstorm associated with hailstorm on 10 Apr 2006
(1138 UTC)
Fig. 15. Thunderstorm on 13 Apr 2006 (1122 UTC) Fig. 16. Thunderstorm on 12 Apr 2007 (1238 UTC)

15. PRADHAN et al. : THUNDERSTORM PREDICTION USING DWR 313
Fig. 17. Hailstorm on 06 May, 2007 (1002 UTC)
A hailstorm was reported by met observatory Sriniketan
(about 170 km NW of Kolkata) at 1132 UTC of hail size
2 cm for 3 minutes.
(d) Another severe thunderstorm occurred on 13th
April
during 0812-1322 UTC (5 hrs 10 min) from NW
direction. Dumdum and Alipore observatories reported
wind squalls of magnitudes 65 km/hr and 72 km/hr at
1225 and 1244 UTC respectively. DWR images showed
that the same system has generated these wind squalls
during its path from NW to SE direction.
(e) A multiple cell thunderstorm of moderate intensity
occurred on 22nd
April and travelled through NW
direction during 0910-1410 UTC. Alipore and Dumdum
recorded the wind squalls of the magnitude 70 km/hr & 48
km/hr respectively at about 1300 UTC. This is also seen
from Table 11 that a hailstorm was reported by Dumdum
at 1230 UTC of hail size 0.7 cm in diameter and lasted for
5 minutes. Above observations show that the system was
very strong and had a potential of generating hailstorm
and wind squalls.
(f) Another severe thunderstorm was developed on 24th
April at 0827 UTC in the NW sector and traveled in SE
direction and lasted for 5 hrs till 1327 UTC. During its
journey, it generated a wind squall at Bankura of
magnitude 56 km/hr at 1032 UTC, another wind squall
observed by Dumdum at 1255 UTC of magnitude
76 km/hr and one more observed by Alipore at 1308 UTC.
4.3. Analysis of thunderstorms during 2007
The analysis of 20 moderate and severe
thunderstorms during the year 2007 shows that there was
only one thunderstorm (severe) occurred in March,
whereas 14 thunderstorms (2 severe and 12 moderate)
occurred in April and 5 thunderstorms (one severe and 4
moderate) in May. Severe thunderstorms during 2007
have been analysed in Table 12. It is revealed that
although CAPE value on 09 April, 2007 was only 2809
J/kg, there were two multiple cell thunderstorm systems
formed after 1200 UTC. One thing may be noted that
0000 UTC observations of convective indices mainly
indicate the generation of a system before 1200 UTC,
however, those developed in the later part are not
predicted very well by convective indices at 0000 UTC.
The first system developed around 1444 UTC in NW
sector moved towards SE direction lasted for 4 hrs 20 min
and converted in to a squall line formation before
dissipation and crossed over Kolkata. However, second
system developed at 1704 UTC survived for about 6 hrs
20 min and it passed to the south west part of Kolkata.
The convective indices at 0000 UTC on 27 April, 2007
predicted the development of moderate thunderstorm and
the day experienced three moderate thunderstorms in
different sectors. Two of these thunderstorms were formed
at same time around 0830 UTC whereas the third one was
developed at 0730 UTC (Table 13). However, the third
was developed locally in SE sector near Kolkata
and lasted for about 5 hrs. Table 13 shows that convective
Reflectivity Scale

16. 314 MAUSAM, 63, 2 (April 2012)
TABLE 13
Moderate thunderstorms during 2007 and convective indices
Date CAPE CINE LI VGP BRN Time & sector of
initiation (UTC)
Time of dissipation
(UTC)
Duration/Direction of
movement
09 Apr 2007 2809 -19 -11.2 0.545 98 0946/ NW 1126 2 hrs 40 min /SSE
11 Apr 2007 3074 -54 -8.8 0.329 131 1024/ NE 1324 3 hrs/SE
13 Apr 2007 (12 Z) 5029 -1 -11.0 0.377 129 0926/ NW 1246 3 hrs 20 min/SE
27 Apr 2007 (00 Z)
(12 Z)
1929
2854
-84
-26
-8.8
-7.8
0.338
0.527
52
26
0830 /NW
0830/ SW
0750/Local
1310
1310
1310
4 hrs 40 min/ SE
4 hrs 40 min/E
5 hrs 20 min/N
28 Apr 2007 (00 Z)
(12 Z)
1627
2589
-3
-28
-8.8
-7.8
0.341
0.37
64
182
0710 /SE 1210 5 hrs/E
29 Apr 2007 2236 -140 -7.7 0.423 232 0910/ SW 1110 2 hrs/SE
30 Apr 2007 2359
2943
-104
-41
-7.0
-8.1
0.235
0.318
144
143
0840/ NW 1310 4 hrs 30 min/SE
02 May 2007 2137 -108 -8.6 0.246 243 0829/Local 1029 2 hrs/NE
06 May 2007 2664 -38 -5.8 0.221 - 0801/ SE 1101 3 hrs/NE
16 May 2007 2040 -13 -5.3 0.142 313 0429/ SW 0829 4 hrs/E
17 May 2007 2379 -10 -5.6 0.235 587 0646/ NW 1122 4 hrs 36 min/ E
TABLE 14
(Wind squall/hailstorm recorded by the Meteorological observatories during year 2007)
Date Squall time
(UTC)
Station Squall
Speed/Direction
Duration of
squall
Time(UTC)/Duration
of hailstorm
Size of
hailstones
Station
reported
22 May 2007 1236
1606
Jamshedpur
Digha
74km/h(N)
68 km/h(NW)
1 min
2 min
Nil Nil Nil
09 Apr 2007 2110
2128
Alipore
Dumdum
80 km/h(W)
50 km/h(NW)
3 min
1 min
Nil Nil Nil
12 Apr 2007 1245
1315
1500
Alipore
Dumdum
Digha
61 km/h(SSW)
70 km/h(NW)
41 km/h(NE)
2 min
1 min
1 min
Nil Nil Nil
13 Apr 2007 2052 Bhubaneshwar 64 km/h(NE) 2 min 1415 1 cm Jamshedpur
26 Apr 2007 1132 Bankura 59 km/h(NW) 2 min Nil Nil Nil
indices at 0000 UTC and 1200 UTC were very favourable
for moderate thunderstorms.
Fig. 16 shows a typical thunderstorm on April 12,
2007 which was developed in NE sector in Bangladesh
and moved towards Kolkata after traveling a distance of
about 200 km. DWR Kolkata observed the system
initiation at 0644 UTC and subsequent radar pictures
showed that the system is moving towards west. The
movement was very unusual as most of the thunderstorms

17. PRADHAN et al. : THUNDERSTORM PREDICTION USING DWR 315
during pre-monsoon period are generated in the western
sectors of GWB region and move towards South-East or
Easterly direction. The radar reflectivity was more than
55 dBZ and the vertical extent was exceeding 16 km. As
the movement of the system was quite steady and straight
(as seen from DWR images), it was highly expected that
the system will pass through north of Kolkata. On the
basis of above observations, an alert of severe
thunderstorm within next two hrs was issued from Met.
Office Kolkata at 1130 UTC to Air Traffic control when
the system was about 80 km away from Kolkata. Ignoring
the warning issued from IMD, an aircraft from a domestic
airline took off around 1300 UTC from the airport and
within minutes it suffered a severe hail strike on the front
windscreen and got severely damaged. The aircraft had to
come back for emergency landing.
Fig. 17 shows a hailstorm occurrence over Kolkata
on 06 May, 2007 at 1002 UTC. The size of hails was
about 2 cm and the hailstorm lasted for 10 minutes.
Authors found some typical signature of the system in the
DWR observations as a very long “Anvil” ahead of the
main cloud and extended up to a distance of 100 km
towards east of Kolkata. Vertical extent of the cloud was
only 8 km and the radar reflectivity was lying in the range
of 53.3 dBz to 56.7 dBz. Authors have taken up a separate
study for such cases of hailstorms using DWR images and
the results are being analysed.
4.3.1. Wind squall and hailstorm during year 2007
Tables 12 & 13 depict the occurrence of severe and
moderate thunderstorms within 300 km radius from
Kolkata during the year 2007 and Table 14 shows the
occurrence of wind squalls and hailstorms reported by the
meteorological observatories. Analysis of some of the
coincident events is done in the following paragraph.
(a) On 22nd
March, a moderate thunderstorm arrived
from NW and travelled through SW sector of Kolkata and
lasted for 5 hrs during 1216 UTC to 1717 UTC. Wind
squall was reported by M.O. Jamshedpur at 1236 UTC at
a speed of 74 km/hr from North direction and by M. O.
Digha at 1606 UTC of magnitude 68 km/hr from NW
direction.
(b) On 09th
April two severe thunderstorms occurred in
the NW sector lasted for 4 hrs 20 min during 1444-1904
UTC & 6 hrs 20 min during 1704-2324 UTC respectively
and arrived Kolkata from NW direction. One moderate
thunderstorm was also developed in the NW sector and
lasted for 2 hrs & 40 min during 0946-1126 UTC. Wind
squalls were reported by Meteorological Offices at
Alipore and Dumdum at 2110 & 2128 UTC respectively
at wind speed 80 km/hr (W) & 50 km/hr (NW). It is
observed that these wind squalls were associated with the
second severe thunderstorm which was lasting in the
duration 1704-2324 UTC.
(c) On 12th
April two severe thunderstorms were
developed in the NE and SW sectors (Table 12) and
travelled towards Kolkata from opposite directions and
merged to a single thunderstorm. The merged cells in the
form of a single thunderstorm further moved in SE
direction. Met. offices at Alipore, Dumdum and Digha
reported wind squalls (Table 14) generated by this system.
(d) Another severe thunderstorm occurred on 26th
April
during 0826-1226 UTC (4 hrs) from NW direction.
Bankura observatory reported wind squalls of magnitudes
59 km/hr at 1132 UTC on its way towards Kolkata.
5. Results
(i) The analysis of 70 thunderstorms in 2005 and related
convective indices and their validation by the analysis of
34 thunderstorms in 2006-2007 showed that Doppler radar
is capable of predicting the development and speed of
movement of the thunderstorms for the next 1-2 hrs using
reflectivity and radial velocity images. However, if the
convective indices of 0000 UTC and 1200 UTC are also
evaluated and used with DWR images then a better now
casting of at least 2-3 hrs may be done with a sufficiently
good accuracy.
(ii) The role of VVP_2 has been established by the
DWR in predicting the genesis and expected movement of
the system once the cloud development is observed in the
radar images.
(iii) Max_Z image has provided much valuable
information in understanding the severity of the system by
observing the vertical extent of the clouds and associated
radar reflectivity. In this study authors have found a very
good correlation between the severity of the system, the
vertical extent and the radar reflectivity. The larger be the
values of these factors, the more is the severity of
thunderstorms. So far, this is the qualitative result of the
analysis but more studies in this area may provide some
formulation to evaluate quantitatively the intensity of
rainfall and severity of the system. The horizontal extent
of the system gives an idea for expected rainfall over an
area in next few hours and to understand the structure of
the cloud system (single cell, multiple cell, bow echo or
squall line).

18. 316 MAUSAM, 63, 2 (April 2012)
(iv) The convective indices proposed in Tables 6 & 7
decide the probability of thunderstorm development and
their severity. Severe thunderstorm will be characterized
by heavy rainfall of the order of 50-100 mm/hr and
moderate thunderstorms provide rainfall in the range of 20
to 50 mm/hr. (The rainfall has been verified from the
observations of the self recording rain gauges in a separate
study undertaken by the authors).
6. Nowcasting technique
Based on above analysis of thunderstorm
occurrences and corresponding convective indices,
following procedure has been suggested for nowcasting of
thunderstorms :
(i) Convective indices CAPE, CINE, LI, VGP and BRN
from 0000 UTC upper air data to be observed.
(ii) Thunderstorm detection to be monitored from DWR
observations in the range of 300 km using PPI_Z &
Max_Z pictures. The radar volume scan may be
performed at an interval of every 10 minutes in automatic
mode.
(iii) VVP_2 product to be observed for the wind direction
and speed around Kolkata. If the wind direction at lower
levels (up to 0.9 km) is either Southerly or South-Easterly
then moisture incursion takes place over this area from
Bay of Bengal and the chances of formation of
thunderstorm are very high (Fig. 16). PPI_V product also
suggests the wind surrounding the thunderstorm but its
interpretation is more complicated than VVP_2.
Moreover, the speed of wind at different levels also plays
an important role in the development of thunderstorm.
Southerly winds of the order of 15-20 knots at levels 0.3 -
0.6 km before 0600 UTC provide the moisture in Chhota
Nagpur region at a faster rate and the chances of severe
thunderstorm development are more between 0800-1000
UTC. If the wind direction at upper levels (3.6-5.0 km) is
northwesterly or westerly then the movement of the
thunderstorm will be towards Kolkata. Authors have
found from the analysis of more than 100 cases that winds
at levels 3.6 to 5.0 km are the “Driving Winds” of the
system because all thunderstorms move according to the
direction of these winds.
(iv) Emphasis should be given for single/multiple cell
system generated in northwest/southwest sectors, as the
time taken by a system to arrive at a place depends upon
the type of thunderstorm, sector of initiation and the
distance. For example, a single cell thunderstorm at a
distance of more than 100 km from Kolkata will have
almost no chance to reach Kolkata however, a multiple
cell system or squall line may reach Kolkata from a
distance more than 200 km.
(v) If the values of convective indices mentioned above
lie within the suggested range and initiation of
thunderstorm is seen through DWR observations then
using PPI_V product, wind direction and speed may be
estimated for movement of the system. Estimation for
their arrival/non-arrival towards a particular area may be
done and warning for high wind speed/ wind squall and
heavy rainfall may be issued accordingly.
7. Conclusions
The products Max_Z, PPI_Z, PPI_V and VVP_2
have been found very useful in the prediction of the
thunderstorm movement over a place with a high degree
of precision in space and time. The convective indices on
the other hand indicate a probability of development of a
system. As VVP_2 product provides a vertical profile of
horizontal winds up to 7.5 km round the clock at a very
short interval, it can be used to estimate wind
direction/speed any time around Kolkata in the range of
50 km and also as a supplementary to the winds derived
from RS/RW.
Convective indices generated from 0000 UTC upper
air observations are basic indicators for most of the storm
formations before 1200 UTC. However, 1200 UTC
indices also play crucial role to determine the atmosphere
being conducive for thunderstorm development after 1200
UTC. (19, 20 March & 03, 05, 22 May during the year
2005 and 08 April, 2006, 13, 27, 28 April, 01, 03, 16
May, 2007). CINE is the next most significant convective
parameter in addition to CAPE, whose large value does
not support formation of thunderstorm even if CAPE and
other parameters are favourable.
Authors have found that very high value of CAPE
exceeding 6000 J/kg and CINE exceeding 200 J/kg are not
favourable for the genesis of thunderstorm in Gangetic
West Bengal region during pre-monsoon period. Very
high values of BRN (>400) and LI (> -10, magnitude
wise) do not support the formation of a thunderstorm.
8. Suggestions
The study reveals that in most of the days during pre-
monsoon period, thunderstorm initiation takes place

19. PRADHAN et al. : THUNDERSTORM PREDICTION USING DWR 317
around 0600 UTC in the Chhota Nagpur region and their
subsequent arrival occurs after 0900 UTC in the
surroundings of Kolkata. So, the convective indices in this
region at 0660 UTC may provide better information about
the favourable conditions for initiation of thunderstorm
and their subsequent motion. Presently, no RS/RW flight
is conducted at 0600 UTC from any station in India, it is
suggested that an additional RS/RW flight may be
performed on the experimental basis and similar studies
may be conducted for better nowcasting of such severe
weather events in Gangetic West Bengal region.
9. Future scope of the work
Nowcasting technique as suggested may need further
studies in the real time mode for more year’s data for
improved and better prediction. As more Doppler Weather
Radars are being installed in various parts of India by
IMD and many more are expected in the near future, this
study will provide guidance to other scientists of IMD for
improvement in the accuracy of nowcasting and extension
of prediction period.
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